In recent years, requirements for the capabilities of a color printing paper, such as high sensitivity, processing stability, high-quality image and rapid processability in the development processing step are increasing. On the other hand, as a result of recent popularization of laser scanning exposure apparatuses, suitability for short-time and high-illuminance exposure is one of the important capabilities. The laser scanning exposure is advantageous in that high-speed exposure is attained and the resolution is improved. However, for applying this to a color printing paper, suitability for unusually very short-time (specifically, 10.sup.-6 second) and high-illuminance exposure is required.
In order to improve the reciprocity law failure of the silver halide emulsion at such high-illuminance exposure, a method of doping a metal compound represented by iridium to the base grain is well known in the art.
The improvement of the reciprocity law failure of the silver halide emulsion by iridium is described, for example, in B. H. Carroll, Iridium Sensitization: A Literature Review of Photographic Science and Engineering, Vol. 24, No. 6 (1980), and R. S. Eachus, The Mechanism of Ir3+ Sensitization (International Meeting 1982 of Photographic Science).
On the other hand, it is also known that the silver halide emulsion having added thereto iridium has a very undesired characteristic such that the photographic capabilities (for example, sensitivity, gradation) change in the time passing after the exposure until the processing. This characteristic is described in H. Zwicky, On the Mechanism of the Sensitivity Increase With Iridium in Silver Halide Emulsions of The Journal of Photographic Science, Vol. 33, pp. 201-203 (1985). According to the methods hitherto proposed, the high-illuminance reciprocity law failure is surely improved but the sensitivity very greatly changes due to changing in the time after exposure until processing and the practical use is not expected at all.
A silver halide emulsion having a high silver chloride content has a purpose of rapid processing in the color development but is deficient in that high-sensitivity high-contrast gradation cannot be obtained by usual chemical sensitization. A large number of attempts have been made to achieve high sensitivity of a high silver chloride emulsion. Among those, a technique of forming a silver bromide-rich phase in the vicinity of the grain apex of a silver halide host grain to thereby achieve high sensitivity is disclosed in JP-A-64-26837 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, JP-A-5-61136 discloses a technique of forming a silver bromide-rich phase in the vicinity of the grain apex of a silver halide host grain through multiple stages. According to these techniques, however, the high-illuminance reciprocity law failure is not improved. U.S. Pat. Nos. 5,284,745, 5,391,471, 5,415,991, 5,043,256 and 5,627,020 disclose a method of doping a metal compound represented by Ir to the inside of a silver bromide-rich phase of a high silver chloride base grain. Further, European Patent Publication 0568091A, U.S. Pat. No. 5,356,770 and JP-A-6-35147 disclose a method of adding a bromide to a high silver chloride grain simultaneously with or after the addition of iridium. According to these methods, the change of sensitivity due to changing in the time after exposure until processing is suppressed and the high-illuminance reciprocity law failure is improved, however, the effect is still not sufficient in the case of a high-illuminance exposure for a very short time, such as laser scanning exposure. In addition, reduction in the sensitivity occurred on exposure at a high humidity is not prevented.